Organic electrolyte for magnesium primary cells



United States Patent M 3,258,368 ORGANIC ELECTROLYTE FOR MAGNESIUMPRIMARY CELLS John L. Robinson, Freeland, and Peter F. King, Midland,

Mich., assignors to The Dow Chemical Company, Midland, Mich., acorporation of Delaware No Drawing. Filed May 20, 1963, Ser. No. 281,7883 Claims. (Cl. 136100) This invention relates to a novel electrolyte foruse in magnesium primary cells and more particularly is concerned withan aqueous carboxylic acid salt electrolyte for such cells.

Aqueous solutions of inorganic salts, e.g. halides, sulfates andperchlorates, long have been used as electrolytes for primary cellswhich utilize magnesium as the anode. (The term magnesium as used hereinis meant to include magnesium and magnesium base alloys containing atleast about 70 weight percent magnesium.) These electrolytes provide aworking potential of the magnesium anodeversus saturated calomelelectrode of from about 1.3 to about 1.5 volts. However, when employedin a magnesium primary cell, the useful or energy producting reaction isaccompanied by a wasteful anode corrosion reaction. As a result, withsuch electrolytes the magnesium anode operates at a maximum coulombicefiiciency of from about 65 to about 75 percent. Also, with theseelectrolytes, the corrosion pattern is non-uniform, i.-e. rough andpitted. Therefore, a considerable excess anode metal thickness is neededto prevent anode perforation during useful cell life. In addition to theloss of a considerable portion of the magnesium anode, the corrosionreaction consumes water such that in the case of dry cells excess watermust be employed in the cathode formulation. Further, hydrogen isevolved at such rates that special venting must be provided inconstruction of the cells. Also, the magnesium hydroxide formed by thiswasteful reaction deposits adjacent the anode and occupies'almost twicethe volume of magnesium consumed. In dry cells, therefore, thisanticipated product build-up must be overcome by providing special celldesign and construction to provide for controlled expansion orcontainment of the cell volume.

When chromic acid electrolytes are utilized in magnesium primary cells,the wasteful corrosion reaction during operation can be held to aminimum and operating anode efficiencies of 90 percent or more obtained.With this latter electrolyte, however, the working potential of themagnesium anode versus the saturated calomel electrode is from 0 toabout O.l0 volt. Thus over 1.2 volts of active working anode potentialis sacrified to obtain the high anode efficiency with the chromic acidelectrolytes.

Now, unexpectedly it has been found that both high anode efficienciesand high working potentials are realized in primary cells employing amagnesium anode when alkali metal and alkaline earth metal salts ofcertain aliphatic and aromatic carboxylic acids are employed as theelectrolyte in such cells.

It is a principal object of the present invention to provide anelectrolyte for primary cells employing a magnesium anode which giveshigh anode efiiciency and high working potentials.

It is also an object of the present invention to provide an electrolytefor use in a magnesium anode containing primary cell the use of which isaccompanied by a desirable uniform corrosion pattern of the anode duringoperation and wherein corrosion products produced in the cell resultsubstantially completely from the useful electrical power producingreaction.

, 3,258,358 Patented June 28, 1966 It is a further object of the presentinvention to provide an electrolyte for magnesium primary cells whichsubstantially eliminates hydrogen generation during cell operation.

It is another object of the present inventon to provide a magnesiumprimary cell having an electrolyte in which there is substantially noheat generated from the wasteful chemical corrosion reaction.

These and other objects and advantages readily will become apparent fromthe detailed description presented hereinafter.

In accordance with the present invention, high anode efficiencies andworking potentials are realized for magnesium primary cells employing anaqueous solution of an alkali metal or alkaline earth metal salt ofcertain aliphatic carboxylic or aromatic carboxylic acids as anelectrolyte. The term alkali metal as used herein is meant to includelithium, sodium, potassium, rubidium and cesium and the term alkalineearth metal as used herein is meant to include magnesium, calcium,strontium and barium.

In general, the aliphatic carboxylic acid salts which preferably areemployed as electrolytes for magnesium primary cells contain from 1 toabout 5 carbon atoms in the carbon chain. Further, these salt formingacids are either saturated or unsaturated and straight or branchedchain. Saturated aliphatic acids which are either monoorpolycarboxylated are suitable for use in the present invention providedthere is at least one methylene or methyl radical or group present foreach carboxy group (COOH) in the parent acid. The terms methyl group ormethyl radical and methylene group or methylene radical" as used hereinmeans (CH and respectively as well as substituted groups wherein one ormore of the H atoms have been replaced by another group. To illustrate,the salts of malonic acid and of hydroxy substituted acids such ascitric acid, glycolic acid, tartaric acid and malic acid, for example,are suitable for use as electrolytes with magnesium cells. Additionally,salts of unsaturated aliphatic carboxylic acids having at least onecarboxylate group for each group also can be employed. However,substituent groups which are strong polarizers such as halo, nitro,phenolic, etc., cannot be used.

In general, it has been found with these aliphatic carboxylic alkalimetal and alkaline earth metal salt electrolytes that cell efficiencyincreases slowly with increasing carbon chain length. This increase isoffset, however, by a corresponding rise in polarization and decrease inthe active potential with increase in carbon chain. With carbon chainsof longer than about 5 carbon atoms detrimental polarization and reducedelectrolyte solubility may occur.

Aromatic carboxylic acid salts suitable for use as electrolytes arethose materials having two or more carboxy groups present on each ringof the acid. Illustrative members include, for example phthalic acid,isophthalic acid and pyromellitic acid. The aromatic acids also cancontain other substituent groups which are not strong polarizers.

With magnesium primary cells, ordinarily the aqueous electrolyteconcentration is from about 0.5 to about 5 normal, based on thecarboxylate anion. Preferably the electrolyte concentration ranges fromabout 1 to about 4 normal. For a specific primary cell construction,generally the present novel electrolyte is employed in about the sameconcentration as the inorganic halide electrolyte normally employedtherein. Further, with a drytype cell, chromates or other traditionalinhibitors can be used in conjunction with the present organicelectrolytes to extend shelf storage life (i.e., open circuit corrosioncontrol) of the cell.

The electrolytes of this invention are used in a conventional manner ina magnesium primary cell as the electrolyte is adapted to any primarycell construction employing a magnesium anode or anodes. In cellconstruction, however, with the present novel electrolyte there is noneed to use excess anode above theoretical as is needed withconventional halide electrolytes which give irregular non-uniformcorrosion with pitting. Also, elaborate gas venting systems are notrequired in cells employing the present novel electrolyte.

The following examples will serve to further illustrate the presentinvention but are not meant to limit it thereto.

EXAMPLE 1 A number of electrochemical tests evaluating the presentelectrolyte were carried out in a standard one-compartment electrolysiscell having a magnesium alloy anode having a nominal composition ofabout 1.8% A1, 1.3% Zn, 0.15% Ca, balance Mg suspended between one inchsquare parallel platinum cathodes. The cell volume was about 200 cubiccentimeters. The nominal anode dimensions were 1.25 inches x 0.25 inch x0.146 inch to give an area of approximately 1.05 square inches (i.e.,6.8 sq. cm.). A Luggin capillary probe was inserted next to the anodeand the anode potential vs. saturated calomel was recorded with either aBrown electronic recorder or a Sanborn oscillograph. Constant currentswere achieved with 24 to 600 volt power sources in series with largeswamping resistors.

Both anode efiiciency and operating anode potentials were determined.

For the efficiency measurements, anode weight losses were determinedafter passage of approximately 2 ampere minutes at various impressedcurrent densities. The Working potentials also were measured at variousimpressed current densities.

Table I, which follows, presents the results of electrical potentialmeasurements and anode efiiciencies obtained with a number ofelectrolytes at various applied currents.

before. The anodes were about 1.5 inches high by about from 0.022 toabout 0.025 inch thick and weighed from about 3.8 to about 4.1 grams.The cells had about 30 hours total operating life to an 0.75 volt endvoltage through 10 ohms resistance. A batch of cathode mix wasformulated by dry blending about 850 grams of African MnO 30 grams BaCrOand 120 grams Shawinigan acetylene black. The dry blend was wetted withabout 530 milliliters of a predetermined electrolyte per 1000 grams ofdry mix. An amount of the resulting wet cathode mix ranging from about48 to about 52 grams was used in each cell. Cells were dischargedcontinuously at a temperature of about 70 F. through 10 ohms resistanceto an 0.75 volt end voltage. Table II summarizes the results of a numberof tests made with magnesium acct-ate electrolyte of variousnormalities. For control purposes, a cell similarly constructed butemploying conventional 3 normal MgBr also Was subjected to the sametest.

Table II Electrolyte Ccll Anode Run No. Normality Capacity, Efliciency,

hrs. percent 0.5 14 82 1. 0 21. 5 83 1. 4 26.0 86 2.0 30.0 88 2. 5 31.087 3.0 27. 5 86 4.0 25.0 85 5.0 4. 5 G7 9 6.0 2. 0 62 10 (control) 3 NMgBr 23 04 EXAMPLE 3 Cells were prepared as described in Example 2except that an aqueous solution of 0.9 N magnesium pyrorncllitate or 1.3N magnesium isophthalate was used as electrolyte. Discharging thesecells through a 10 ohm drain to 0.75 end voltage resulted in anodeefiiciencies of 80% for the magnesium pyromellitate and 79% for themagnesium isophthalate electrolytes respectively.

EXAMPLE 4 Cells having an operational life of from about 140- 150 hoursto 1.0 volt through a 50 ohm drain were prepared using the sameconstruction as described in Example 2 except that the cathode mix wasabout 82% TABLE I Potential vs. Satd. Calomcl Anode Efficiency (v0lts)(percent) Run Normality No. Electrolyte Solute Aqueous Applied CurrentApplied Current,

Electrolyte Milliamperes Milliamperes Magnesium Acetate 1. O 1. 1. 43 1.43 82 89 Potassium Acetate... 1. 0 1. 52 1. 44 1. 40 83 87 CalciumAcetate 1. 0 1. 45 1. 43 1. 42 83 87. 5 Magnesium Propionate. 1. 0 1.39 1. 32 1. 16 81 88. 5 Calclum Malonate- 1. 0 1. 1. 45 1. 45 67. 5 07.5

Sodium Suceiuate 1. 0 1.46 1. 40 1. 40 81.5 80 Calcium Acrylat 1. 0 1.35 1. 30 1. 25 84 90. 5

Sodium Maleate. 1. 0 1. 43 1. 34 1. 39 80 88 Sodium Malatc.. 1. 01.40 1. 37 1. 27 90. 5 92 Potassium Tartrate. 1. 0 1. 49 1. 35 1. 12 90.5 95. 5 11 Magnesium Glycolate. 1. 0 1.48 1.47 1.39 83 91. 5 12----Trisodluln Citrate 1. 0 1. 51 1. 45 1. 28 94. 5 96. 5

13.... Dipptassium Phthalate... 1. 0 1. 50 1. 37 1. 25 89. 5 93 14....Sodium Adipate 1. 0 1. 1. 49 1. 32 86. 5 97 15 Sodium Pimelate 1. 01.44 1. 33 91 16.... Disodium Isophthalate. 1. 0 1. 48 1. 37 1 12 92 92.5

17.... Tetrasodium Pyromellitat 1. 0 1. 45 1. 38 1. 27 92 93 18.-.-Disodium Malate 1. 0 1. 41 1. 36 1. 26 90 93 EXAMPLE 2 Steel jacketed Dsize primary cells were fabricated in accordance with the cellembodiment described and set forth in U.S. 2,845,471. These cellsutilized AZ21X1 MnO 3% BaCrO 15% acetylene black Wet with 675 cubiccentimeters of 3 N magnesium acetate electrolyte per 1000 grams of thedry cathode blend. These cells were subjected to a continuous drainthrough 50 ohm magnesium alloy anodes of composition set forthhereinresistance to a 1.0 volt end voltage.

Control cells employing 3 N magnesium bromide electrolyte were similarlyprepared and tested. Tabl III shows the results of these tests.

Table 111 Cell Anode Run Anode Material Electrolyte Capacity, Elfi- N 0.Hrs. eiency,

Percent 1 AZ21X1 alloy". Magnesium acetate-. 140 76 2 .d0 MgBr(eo11trol) 142 62 A second series of similar cells was prepared whereinhigh purity magnesium was utilized as anode material instead of thealuminum-zinc containing magnesium alloy. These cells were tested to a1.0 volt end voltage through a ohm resistor.

Table IV presents the results of this run.

Various modifications can be made in the present invention withoutdeparting from the spirit or scope thereof for it is understood that wlimit ourselves only as defined in the appended claims.

We claim:

1. In a magnesium battery having a high anode elficiency and potentialwhich includes a magnesium anode, a cathode, an aqueous electrolyte, achromate inhibitor and a current collector, the improvement whichcomprises; providing an aqueous carboxylic acid salt electrolyte thesolute of which is a member selected from the group consisting of alkalimetal and alkaline earth metal salts of aliphatic and aromaticcarboxylic acids and further charactertized in that the aliphatic acidshave a carbon chain length of from 1 to about 5 carbon atoms and thereis at least one radical selected from the group consisting of methyleneand methyl present for each carboxy group of said aliphatic acid and thearomatic carboxylic acid salts have at least two carboxy groups presenton each ring of said aromatic acid, and wherein the solute concentrationin said electrolyte ranges from about 0.5 to about 5 normal.

2. The magnesium battery as defined in claim 1 wherein the solute of theaqueous carboxylic acid salt electrolyte is magnesium acetate and theconcentration of said magnesium acetate in said electrolyte ranges fromabout 1 to about 4 normal.

3. The magnesium battery as defined in claim 2 having a manganesedioxide cathode and a carbon current collector.

References Cited by the Examiner UNITED STATES PATENTS 2,245,528 6/1941Loder 260-524 2,993,946 7/1961 Lozier 13690 3,057,944 10/1962 Ruetschiet al. 136-154 X 3,095,331 6/1963 Davis 136--154 X WINSTON A. DOUGLAS,Primary Examiner.

B. J. OHLENDORF, Assistant Examiner.

1. IN A MAGNESIUM BATTERY HAVING A HIGH ANODE EFFICIENCY AND POTENTIALWHICH INCLUDES A MAGNESIUM ANODE, A CATHODE, AN AQUEOUS ELECTROLYTE, ACHROMATE INHIBITOR AND A CURRENT COLLECTOR, THE IMPROVEMENT WHICHCOMPRISES; PROVIDING AN AQUEOUS CARBOXYLIC ACID SALT ELECTROLYTE THESOLUTE OF WHICH IS A MEMBER SELECTED FROM THE GROUP CONSISTING OF ALKALIMETAL AND ALKALINE EARTH METAL SALTS OF ALIPHATIC AND AROMATICCARBOXYLIC ACIDS AND FURTHER CHARACTERITIZED IN THAT HTE ALIPHATIC ACIDSHAVE A CARBON CHAIN LENGTH OF FROM 1 TO ABOUT 5 CARBON ATOMS AND THEREIS AT LEAST ONE RADICAL SELECTED FROM THE GROUP CONSISTING OF METHYLENEAND METHYL PRESENT FOR EACH CARBOXY GROUP OF SAID ALIPHATIC ACID AND THEAROMATIC CARBOXYLIC ACID SALTS HAVE AT LEAST TWO CARBOXY GROUPS PRESENTON EACH RING OF SAID AROMATIC ACID, AND WHREIN THE SOLUTE CONCENTRATIONIN SAID ELECTROLYTE RANGES FROM ABOUT 0.5 TO ABOUT 5 NORMAL.